1. Field of the Invention
The present invention generally relates to tuning-fork type vibratory gyros, and more particularly a tuning-fork type vibratory gyro having a piezoelectric substance.
2. Description of the Related Art
A gyroscope has been used to determine the current position of a vehicle such as an airplane, ship or a satellite. Recently, a gyroscope has been applied to devices for personal use, such as car navigation and vibration detection in video cameras and still cameras.
A conventional coma gyro detects an angular velocity by utilizing a principle such that a coma (disk) which is rotating continues to rotate without any change of the attitude thereof while keeping the rotation axis even when a device equipped with the coma gyro is tilted. Recently, an optical type gyro or a piezoelectric type gyro has been developed and has come into practical use. The principles of the piezoelectric type gyro were proposed around 1950. Various piezoelectric type gyros having, for example, a tuning-fork, a cylinder or a semi-spherical member have been proposed. Recently, a vibratory gyro having a piezoelectric member has been in practical use. Such a vibratory gyro has less measurement sensitivity and precision than those of the coma gyro and the optical gyro, but has advantages in terms of size, weight and cost.
A conventional vibratory gyro having a piezoelectric substance utilizes the fact that the Coriolis force takes place in a direction perpendicular to a vibration of an object when an angular velocity is applied to the vibrating object. The principle of such a piezoelectric type vibratory gyro can be analyzed as a dynamic model (see, for example, ELASTIC WAVE DEVICE HANDBOOK, Ohmu Sha, pp. 491-497). Various types of piezoelectric type vibratory gyros have been proposed. For example, the above handbook shows a Sperry tuning-fork gyro, a Watson turning-fork gyro, a vibrating reed gyro and a cylinder type vibratory gyro.
Yet another type of a piezoelectric type gyro using a vibrating reed (flexural mode vibrator) made of a single crystal of LiTaO.sub.3 has been proposed (see, for example, Konno et al., "EXPERIMENTS ON ANGULAR RATE SENSOR USING LiTaO.sub.3 FLEXURAL MODE VIBRATOR", 1986 Autumn National Convention Record, The institute of Electronics and Communication engineers of Japan, P1-79). Further, Japanese Laid-Open Patent Application No. 61-294311 discloses a tuning-fork type gyro using piezoelectric ceramics. The gyro proposed in the above application uses a vibrator having two arms respectively having different polarizations so as to function as a driving arm and a detection arm, respectively.
The above piezoelectric type gyro having the LiTaO.sub.3 single crystal has a little loss and good temperature performance. However, this gyro has a symmetrical structure and has almost the same mechanical Q values of the two orthogonal modes. The response performance as a sensor depends on the mechanical Q value of a piezoelectric element. Hence, it is not desirable that the piezoelectric element be driven with a high Q value. If the piezoelectric element is driven with a high Q value, the response performance will be degraded, while a vibration due to the Coriolis force to be detected has a high Q value. For example, it is required that a piezoelectric gyro applied to a vibration in hand-carried devices respond to frequencies of tens of heltz. For this requirement, there is an upper limit of the mechanical Q. Further, as a problem encountered when mounting the gyro to a supporting element, it is very difficult to hold the steady point (which corresponds to a node of vibration) of the piezoelectric element with a high precision.
The above-mentioned tuning-fork gyro having piezoelectric ceramic needs a polarizing process having a number of steps for producing the different polarizations, and is therefore expensive. The above-mentioned Laid-Open Japanese application does not provide any useful considerations regarding the gyro mounting method, frequency adjusting and setting of the mechanical Q values of the driving-side and detection-side vibrations.